In storage systems, a plurality of storage devices are provided, and backup is performed by copying data of one storage (business storage) device to another storage (backup storage) device.
Further, remote copy functions of copying data of one storage device to another storage device installed at a geographically separated place among a plurality of storage devices arranged at remote sites have been also known. As one of such remote copy functions, a remote equivalent copy (REC) consistency mode function of guaranteeing writing sequentiality of data when remote copying is performed in an asynchronous mode has been known.
As a storage device, for example, redundant arrays of inexpensive disks (RAID) devices are used. The RAID device is one in which a plurality of hard disk drives (HDDs) are combined and managed as a redundant storage, and provides a higher-level device with a virtual storage (logical volume). The RAID device is a storage system of a distributed cache memory type, and performs a process of reading/writing data of a logical volume in units of controller modules (CMs).
In the REC function, data in a storage medium in a storage device (a copy source device) of a copy source is stored in a recording-dedicated buffer in the copy source device prepared for each CM. Further, the recording-dedicated buffer of each CM is divided into a plurality of areas (buffer areas) and managed in order to guarantee sequentiality of a data reading/writing process. The divided individual buffer areas are managed as generations.
When the buffer areas of the recording-dedicated buffer are fully filled or a certain period of time elapses after data storage starts, the copy source device transmits data in the buffer areas of the same generation to a storage device (a copy destination device) of a copy destination connected via a network together. Hereinafter, in the REC function, a technique of transmitting data of the same generation in the recording-dedicated buffer of the copy source device to a storage device of a copy destination connected via a network may be referred to as a batch transmission scheme.
The copy destination device first stores received data in the recording-dedicated buffer. When all data is received, the copy destination device develops collectively data in the recording-dedicated buffer into a recording medium of the copy destination.
In the REC function, the buffer areas of the same generation on each CM are collectively controlled in units of buffer sets in order to perform copying of a batch transmission scheme guaranteeing sequentiality using the recording-dedicated buffer as described above. Further, sequentiality is guaranteed by developing data from the copy source recording-dedicated buffer in units of buffer sets.
FIG. 21 is a diagram illustrating a conventional buffer configuration of a storage system. In a storage system 500 illustrated in FIG. 21, two storage devices 501a and 501b are connected to each other via a network 502. In the example illustrated in FIG. 21, the storage device 501a is the copy source device, and the storage device 501b is the copy destination device. Hereinafter, the storage device 501a may be referred to as a copy source device 501a, and the storage device 501b may be referred to as a copy destination device 501b. 
Each of the storage devices 501a and 501b includes four CM #0 to #3. In the copy source device 501a, each of the CMs #0 to #3 includes a transmitting buffer. Meanwhile, in the copy destination device, each of the CMs #0 to #3 includes two receiving buffers (recording-dedicated buffers) of a local buffer and a mirror buffer.
Further, in the copy source device 501a, the transmitting buffer of each CM is divided into 8 buffer areas (corresponding to 8 generations). Meanwhile, in the copy destination device 501b, the local buffer and the mirror buffer of each of the CMs #0 to #3 are divided into 8 buffer areas (corresponding to 8 generations), similarly to the copy destination device 501b. 
In the copy destination device 501b, data which is transmitted from the copy source device 501a and transferred from a port (not illustrated) to the local buffer of each CM is duplicated by storing a copy thereof in the mirror buffer of another CM.
For example, in the copy source device 501a, data to be transferred to the local buffer of the CM #0 is duplicated by storing a copy thereof in the mirror buffer of the CM #1. Similarly, data to be transferred to the local buffer of the CM #3 is duplicated to the mirror buffer of the CM #0, and data to be transferred to the local buffer of the CM #1 is duplicated to the mirror buffer of the CM #2. Further, data to be transferred to the local buffer of the CM #2 is duplicated to the mirror buffer of the CM #3.
Here, in the remote copy technique, a technique in which in the copy destination device 501b, when a certain CM is degraded and enters a disable state, a transmission destination of data being transmitted to the local buffer of the corresponding CM is switched to the mirror buffer of another CM to which the data is being duplicated, so that REC consistency transfer is continuously performed is known. In other words, a degrade mirror monitoring technique is known.
For example, in the copy destination device 501b, when the CM #0 has degraded, a process of developing data of the CM #0 is continued using (duplicated) data stored in the mirror buffer of the CM #1. Further, as the CM #0 has degraded, the data of the CM #0 stored in the mirror buffer of the CM #1 becomes a non-duplexing state (a duplexing-broken state).
Further, for data being transmitted to the local buffer of the CM #3, the REC consistency transfer is continued in the duplexing-broken state while causing the data not to be transferred to the mirror buffer of the CM #0 since it is impossible to use the mirror buffer of the CM #0. Hereinafter, a state in which monitoring is being performed so that duplexing is not performed may be referred to as a duplexing inhibition monitoring state.
In the REC consistency transfer, the copy destination device 501b sequentially transmits a buffer release notification and an available buffer notification to the copy source device 501a each time development of data of the recording-dedicated buffer is completed or released.
For example, when the CM #0 has degraded, the copy destination device 501b transmits an ID (a receiving buffer ID) representing a buffer area of an available (alive) buffer other than the buffer of the degraded CM #0 to the copy source device 501a as the available buffer notification.
The copy source device 501a registers a reception side buffer ID informed by the available buffer notification and a reception side buffer ID excluding a reception side buffer ID already allocated by a transmitting buffer to an available buffer management table (not illustrated) at a transmission side.
When transferring data, the copy source device 501a allocates the reception side buffer ID registered to the available buffer management table, and transmits data to the copy destination device 501b. In the copy destination device 501b, when data is received, a CM of a transfer destination of the data is determined with reference to the reception side buffer ID attached to the data.
Meanwhile, in the copy destination device 501b that is in the duplexing inhibition state due to degradation of some CMs, when degradation is solved by replacement of broken CMs or the like, it is necessary to release the duplexing inhibition state. An operation of releasing the duplexing inhibition state and causing the buffer to the duplexing state is referred to as “duplexing reconstruction.”
Japanese Laid-open Patent Publication No. 2006-106883
Japanese Laid-open Patent Publication No. 2006-260292
Japanese Laid-open Patent Publication No. 2007-511844
However, in the conventional storage system, even before duplexing reconstruction, that is, even in the state in which there is a non-duplexed buffer, a CM including a non-duplexed buffer may degrade. When a CM including a non-duplexed buffer is degraded, an error suspend state in which it is impossible to use a REC session is caused.
For example, in the example described with reference to FIG. 21, in the copy destination device 501b, when the CM #0 has degraded and thus in the duplexing inhibition monitoring state, data of the CM #3 is in the duplexing-broken state. In this state, when the CM #3 is further degraded, in the copy destination device 501b, the data of the CM #3 is lost, leading to the error suspend state.
In the error suspend state, it is necessary to re-establish an REC session between the copy source device 501a and the copy destination device 501b, and it is necessary to perform copying of all data of the copy source device 501a as initial copying from the beginning. Thus, there is a problem in that it takes a long time to resume the sequentiality-guaranteed data transfer.